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Computer software for ensembling all the space vehicle (SV) and ground clocks of the Global Positioning Systems (GPS) was implemented at the Master Control Station (MCS) on 17 June 1990. Improved GPS time stability and steering control are predicted. The perceptions of both Air Force and outside users are assessed. Current performance is compared with theoretical predictions.
Estimates of long-term precision and accuracy of interstation vectors using GPS are reported. Precision is estimated from three experiments conducted in central California over a period of 1.2 years. Horizontal precision is 3 to 8 mm for baseline vectors between 100 and 450 km in length, and the vertical precision ranges from 11 to 33 mm. Accuracy is assessed by comparing VLBI estimates with results from six GPS experiments conducted at Mojave and Palos Verdes over a period of 2.2 years. Rates for all vector components agree with VLBI within one standard deviation.
As the development of GPS continues, there will be an increasing need for a software-centered signal model. This model must accurately generate the observed pseudorange that would typically be encountered. The observed pseudorange varies from the true geometric range because of range measurement errors, which stem from a variety of hardware and environmental factors. In this paper, these errors are classified as either deterministic or random, and, where appropriate, their models are summarized. Of particular interest is the model for Selective Availability, which was derived from actual GPS data. The procedure for determination of this model, known as system identification theory, is briefly outlined. The synthesis of these error sources into the final signal model is given, along with simulation results.
Geodetic measurements with Rogue GPS receivers from sites in the California Permanent GPS geodetic Array (PGGA) have been analyzed using the GIPSY orbit-determination and baseline-estimation software. Based on an unbiased selection of 23 daily measurements spanning 8 months, the LF contributions to the long-term repeatabilities of baseline measurements are approximately 5, 3, and 8 mm for the east, north, and vertical components. Short-term contributions to the long-term repeatabilities were evaluated by examining data from the week of October 21, 1990, which showed the lowest short-term scatter. For this week, daily repeatabilities of 2-3 mm in the horizontal and 4 mm in the vertical have been achieved for the 172-km JPL-Pinyon baseline, consistent with carrier phase date noise of about 6 mm. High quality (less than about 5 mm) repeatabilities have been achieved for all components of the other baselines as well.
Proposed scheme for postprocessing of data acquired by receivers monitoring broadcasts from Global Positioning System (GPS) satellites reduces those errors in computed positions of receivers caused by pseudorandom offsets introduced into transmitted signals. Offsets canceled exactly with help of codes available to privileged users and intended to degrade accuracies available to nonpriviledged users, pursuant to policy called "selective availability" (S/A). Proposed scheme devised primarily for GPS receivers aboard TOPEX satellite and at associated ground stations.
It was shown that the use of the Defense Mapping Agency's (DMA) precise ephemerides brings a significant improvement to the accuracy of GPS time transfer. At present a new set of precise ephemerides produced by the National Geodetic Survey (NGS) has been made available to the timing community. This study demonstrates that both types of precise ephemerides improve long-distance GPS time transfer and remove the effects of Selective Availability (SA) degradation of broadcast ephemerides. The issue of overcoming SA is also discussed in terms of the routine availability of precise ephemerides.
A method for canceling the effect of GPS ephemeris error on user position is proposed. In this method, the baseline vectors from the reference stations to the user are estimated without adjusting the GPS ephemeris. The user position is computed by adjustment using differenced data from the user and each station separately and averaging the results with weights inversely proportional to the lengths of the baselines. Alternatively, the differenced data can be averaged in a similar manner before the user position is estimated. The averaging procedure cancels most of the ephemeris error because the error is proportional to the length of the baseline. A numerical simulation is performed to demonstrate and evaluate the method. Two reference stations with perfectly known locations are assumed to be placed several hundred kilometers apart. A user receiver with a poorly known location is located between the stations. The user positions are first estimated separately using data from the user and each station and then averaged. The averaging reduces the error by about one order of magnitude.
The possibilities for providing precise and frequent measurements of earth's orientation in space by using GPS technology are reviewed. In particular, attention is given to the concepts as polar motion and Universal Time, definition of reference frames for unambiguous measurements of earth's rotations, and data processing strategies. Some of the results achieved to date are examined, and it is shown that Universal Time changes can be measured using GPS with an accuracy of better than 100 microseconds over a few hours. Finally, future plans are discussed.
Methods and results for realtime pitch, roll, and heading determination of a dynamic flexible platform are presented. Consideration is given to a GPS attitude and heading determination system installed in Ohio University's Douglas DC-3 research aircraft. Signals from four antennas are processed by an Ashtech 3DF 24-channel GPS receiver. Data from the receiver are sent to a microcomputer for storage and further computations. Attitude and heading data are sent to a second computer for display on a software generated artificial horizon. Demonstration of this technique proves its candidacy for augmentation of aircraft state estimation for flight control and navigation as well as for numerous other applications, such as reliable aircraft state estimation, measurement of structural flexing of large space structures, synthetic aperture radar, aerial photogrammetry, pointing systems, spacecraft attitude control, rendezvous and docking, and road mapping.
Dual-frequency pseudorange and carrier phase data streams can be analytically combined into a single equivalent data stream, reducing the data volume and computing time in the filtering process for parameter estimation by a factor of 2 to 4. The resulting single data stream is that of carrier phase measurements with both data noise and bias uncertainty strictly defined. Based on these analytical formulas the equivalent GPS measurements can be formed by simple and efficient numerical calculations without any degradation in data strength. Formulation for the equivalent GPS measurements and their covariances are given in closed form; and a numerical simulation is performed to demonstrate the validity and effectiveness of the equivalent measurements.
A series of covariance analyses is presented for the Navy's Geosat Follow-On (GFO) altimetric satellite using selective availability/anti-spoof (SA/A-S) qualified (dual frequency) and non-SA/A-S qualified (single frequency) GPS receiver configurations. Data obtained indicate that a SA/A-S qualified GPS receiver is capable of meeting the 10 cm GFO post-processed radial orbit accuracy requirement when using reduced dynamic tracking. The non-SA/A-S qualified receiver is also capable of approaching this 10 cm radial uncertainty level if all systematic errors are removed from the C/A pseudorange and reduced dynamic tracking is used again.
The purpose of this study is to determine the adequacy of the Global Positioning System (GPS) in providing relative navigation for automated rendezvous and proximity operations. The study was performed using the Proximity Operations Simulator (POS), Lockheed's high-fidelity, six-degree-of-freedom simulation of the Space Shuttle and Space Station. This simulation includes identical models of GPS receivers for each vehicle. The navigation software in each vehicle includes identical Kalman filters. Each vehicle estimates its own state, and the relative state is obtained by simply subtracting absolute states.
The purpose of this study is to determine the adequacy of the Global Positioning System (GPS) in providing relative navigation for automated rendezvous and proximity operations. The study was performed using the Proximity Operations Simulator (POS), Lockheed's high-fidelity, 6 degree of freedom simulation of the space shuttle and space station. This simulation includes identical models of GPS receivers for each vehicle. The navigation software in each vehicle includes identical Kalman filters. Each filter computes the absolute state of its vehicle, and the relative state vector is obtained by simply subtracting absolute states.
Aircraft guidance and positioning during the final approach and landing phases of flight requires a high degree of accuracy. The Global Positioning System operating in differential mode (DGPS) is being considered for this application. Prior to implementation, all sources of error must be considered. Multipath has been shown to be the dominant source of error for DGPS. Theoretical studies have verified the severity of multipath within the final approach and landing regions. This paper presents a study of GPS multipath errors during these critical phases of flight. A discussion of GPS multipath error characteristics will be presented along with actual multipath data. The data was collected using P-code and C/A-code receiver architectures. Data was collected onboard a dual-engine fixed-wing research aircraft. Aircraft dynamics are considered in the data analysis.
Mass balance is an integral part of any comprehensive glaciological investigation. Unfortunately, it is hard to determine at remote locations where there is no fixed reference. The Global Positioning System (GPS) offers a solution. Simultaneous GPS observations at a known location and the remote field site, processed differentially, will accurately position the camp site. From there, a monument planted in the firn atop the ice can also be accurately positioned. Change in the monument's vertical position is a direct indicator of ice thickness change. Because the monument is not connected to the ice, its motion is due to both mass balance change and to the settling of firn as it densifies into ice. Observations of relative position change between the monument and anchors at various depths within the firn are used to remove the settling effect. An experiment to test this method has begun at Byrd Station on the West Antarctic Ice Sheet and the first epoch of observations was made. Analysis indicates that positioning errors will be very small. It appears likely that the largest errors involved with this technique will arise from ancillary data needed to determine firn settling.
During April and May of 1991, the Avionics Engineering Center at Ohio University completed the first set of realtime flight tests of a GPS interferometric attitude and heading determination system. This technique has myriad applications for aircraft and spacecraft guidance and control. However, before these applications can be further developed, a number of guidance accuracy issues must be considered. Among these are: signal derogation due to multipath and shadowing, effects of structural flexures, and system robustness during loss of phase lock. This paper addresses these issues with special emphasis on the information content of the GPS signal, and characterization and mitigation of multipath encountered while in flight.
An attempt is made to quantify the contribution of tropospheric refraction, GPS ephemerides errors, and unresolved ambiguities to the overall error in the use of differential GPS to estimate aircraft position over very long distances. The extent to which such errors can be filtered out when estimating a trajectory, and how efficiently this may be done, are addressed.
A new PC-based GPS flight management display system (GFMS) was developed for Greenland ice cap mapping during the NASA Greenland Ice Sheet mapping experiment, when a total of nine flights were made over four different flight tracks, of which two coincided with ground tracks of the ERS altimeter satellite. In this system, the GFMS inputs the GPS position data to a PC, which generates aircraft automatic pilot steering commands and a cockpit display. The display includes (1) the course deviation indicators for cross-track error and altitude, (2) the flight plan and waypoint map overlay oriented to the aircraft, and (3) various other mission-pertinent numerical data.